This invention relates to a composition of matter having relatively low thermal resistivity which is particularly useful for providing a thermally stable environment for buried electrical transmission and distribution equipment, e.g. cables and transformers.
In recent years, because of ecological as well as operational reasons, electrical equipment such as high voltage transmission and distribution power lines, transformers, etc. have been placed underground. One of the most severe limitations on the capabilities of such installations is the dissipation of heat generated by the flow of electrical power therethrough. If the thermal resistivity of the environment surrounding the buried equipment is unsatisfactorily high, the heat generated during functioning of the equipment can cause an increase in the temperature of the equipment which is beyond the tolerable limits thereof, and on extended operation at such temperatures, failure or destruction of the equipment may occur.
For this reason, underground facilities must be typically designed according to their expected thermal environment. Furthermore, since most of the thermal impedance from the heat source, i.e. the electrical conductor, to ambient, i.e. the air, resides in the intervening earth therebetween, the earth becomes an overwhelming factor in calculating equipment size. In such calculations, limitations resulting from non-uniformity of the surrounding earth environment must be taken into account. For example, earth found along the route of an underground cable typically varies widely in heat conducting properties, and as such would require cable sizing compatible with soil areas having the highest thermal resistivity. For this reason, native soil is seldom returned to a transmission cable trench.
To alleviate this situation, present commercial practice dictates the use of a prepared backfill material having known resistivity characteristics to replace the native soil. This backfill material has typically been a well graded soil exhibiting resistivity within a satisfactory range for an assumed thermal history, i.e. duration of expected subsurface temperature and moisture availability. In many cases, however, this backfill must be transported to the construction site, greatly increasing the cost of the project. Furthermore, in many instances, the replaced native soil must be transported from the construction site. As such, the installation costs for the utility are greatly increased.
One solution to the problem of heat dissipation for buried electrical transformers is disclosed in U.S. Pat. No. 3,212,563, wherein jacketed transformers are taught, with cooling water being piped to and from the transformer jacket to dissipate the heat generated during normal operation. While the cooling water does indeed function to dissipate the heat generated, such a design would greatly increase transformer costs and maintenance problems on such a system would tend to render the system commercially infeasible.
Another solution to this problem, as disclosed in U.S. Pat. No. 3,719,511, utilizes a weak mix concrete to encase the electrical equipment therein. There is some difficulty in reentry once the concrete mix is utilized, however, and also some cracking of the concrete system may occur, thereby detracting from its capabilities as an effective back fill material.
Still another backfill material, as defined by U.S. Pat. No. 3,082,111, utilizes a composition having particularly defined percentages of sized sand particles, assertedly to optimize packed density and correspondingly the thermal resistivity. Such a material would still not reduce the costs involved with transporting backfill to the construction site, etc.
It has now been found that soil can be simply treated with a stabilizing agent comprising a clay, preferably a kaolinitic or montmorillonite clay, with a dispersing agent therefor, to produce a backfill composition having improved thermal resistivity properties. Depending on the characteristics of the native soil excavated during trenching operations, in many instances the same soil can be treated and returned as backfill to the trench.